Inverted metamorphic multijunction solar cell

ABSTRACT

A method of manufacturing a solar cell comprising: providing a growth substrate depositing on the growth substrate an epitaxial sequence of layers of semiconductor material forming at least a first and second solar subcells depositing a semiconductor contact layer on top of the second solar subcell depositing a reflective metal layer over said semiconductor contact layer such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850 to 2000 nm depositing a contact metal layer composed on said reflective metal layer mounting and bonding a supporting substrate on top of the contact metal layer and removing the growth substrate.

REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.12/544,001 filed Aug. 19, 2009.

BACKGROUND 1. Field of the Disclosure

The present disclosure relates to the field of semiconductor devices,and to fabrication processes and devices such as multijunction solarcells based on III-V semiconductor compounds including a metamorphiclayer. Such devices in some embodiments are also known as invertedmetamorphic multijunction solar cells.

2. Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to manufacture. Typical commercialIII-V compound semiconductor multijunction solar cells have energyefficiencies that exceed 27% under one sun, air mass 0 (AM0),illumination, whereas even the most efficient silicon technologiesgenerally reach only about 18% efficiency under comparable conditions.Under high solar concentration (e.g., 500×), commercially availableIII-V compound semiconductor multijunction solar cells in terrestrialapplications (at AM1.5D) have energy efficiencies that exceed 37%. Thehigher conversion efficiency of III-V compound semiconductor solar cellscompared to silicon solar cells is in part based on the ability toachieve spectral splitting of the incident radiation through the use ofa plurality of photovoltaic regions with different band gap energies,and accumulating the current from each of the regions.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures. Theindividual solar cells or wafers are then disposed in horizontal arrays,with the individual solar cells connected together in an electricalseries circuit. The shape and structure of an array, as well as thenumber of cells it contains, are determined in part by the desiredoutput voltage and current.

Inverted metamorphic multijunction (IMM) solar cell structures based onIII-V compound semiconductor layers, such as described in M. W. Wanlasset al., Lattice Mismatched Approaches for High Performance, III-VPhotovoltaic Energy Converters (Conference Proceedings of the 31^(st)IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press,2005), present an important conceptual starting point for thedevelopment of future commercial high efficiency solar cells. However,the materials and structures for a number of different layers of thecell proposed and described in such reference present a number ofpractical difficulties relating to the appropriate choice of materialsand fabrication steps.

Improving the efficiency of space-grade solar cells has been the goal ofresearchers for decades. Efficiency of space-grade solar cells hasimproved from 23% (for a dual-junction InGaP/GaAs on inactive Ge) to29.5% (for a triple-junction InGaP/InGaAs/Ge solar cell), which not onlybeen realized through improved material quality, but also throughimproved cell designs that reduce power degradation from chargedparticle radiation and higher temperatures that is characteristic of thespace operating environment.

The composition of the back metal layer in such inverted metamorphicmultijunction solar cells is one such consideration that has notreceived much attention. Although the related application noted abovediscusses a variety of different metal compositions, some suggestedmetals diffuse into the active layers of the adjacent subcell, therebyimpairing its efficiency and efficacy. Moreover, scant attention hasbeen paid to the goal of increasing the optical path length in the solarcell through back reflection, and reducing the temperature of the solarcell through rejecting long wavelength (IR) radiation. The presentdisclosure seeks to address such issues.

SUMMARY

Briefly, and in general terms, the present disclosure provides a methodof manufacturing a solar cell comprising: providing a growth substrate;depositing on the growth substrate an epitaxial sequence of layers ofsemiconductor material forming at least a first and second solarsubcells; depositing a semiconductor contact layer on top of the secondsolar subcell; depositing a reflective metal layer composed of any oneor more of the following metals or alloys thereof: Al, Be, and Ni to athickness between 50 nm and 5 microns over said semiconductor contactlayer such that the reflectivity of the reflective metal layer isgreater than 80% in the wavelength range 850-2000 nm; depositing acontact metal layer composed of one or more layers of Ag, Au and Ti onsaid reflective metal layer; mounting and bonding a surrogate substrateon top of the contact metal layer; and removing the first substrate.

In some embodiments, the reflective metal layer comprises one or more ofthe following metals or alloys thereof: Ag, Al, Au, Be, Cu, Mo, Ni orTi, and further comprising depositing a diffusion barrier layer directlyon said semiconductor contact layer, wherein the reflective metal layeris deposited directly on the diffusion barrier layer.

In some embodiments, the thickness of the reflective metal layer is inthe range of 50 nm to 200 nm.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Ag.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Al.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Au.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Be.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Cu.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Mo.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Ni.

In some embodiments, the composition of the reflective metal layer iscomposed solely of Ti.

In some embodiments, the thickness of the reflective metal layer isbetween 50 nm and 5 microns.

In some embodiments, these further comprise the step of depositing adiffusion barrier layer directly on said semiconductor contact layer,wherein the reflective metal layer is deposited directly on thediffusion barrier layer.

In some embodiments, the diffusion barrier layer is composed of one ormore layers of Cr, Pd, Pt, Si, Ti or TiN.

In some embodiments, the diffusion barrier layer has a thickness ofbetween 0.1 nm and 10.0 nm.

In some embodiments, in the case of Si in the diffusion barrier layer,the silicon may be either polycrystalline or amorphous.

In some embodiments, the diffusion barrier layer has a thickness between1.0 and 3.0 nm.

In another aspect the present disclosure provides a method ofmanufacturing a solar cell comprising: providing a growth substratedepositing on the growth substrate an epitaxial sequence of layers of aIII-V compound semiconductor material forming at least a first top orlight-facing solar subcell and a second bottom solar subcell, the secondbottom solar subcell has a top surface and a bottom surface depositing adiffusion barrier layer directly on the bottom surface of the bottomsolar subcell depositing a reflective metal layer directly on thediffusion barrier layer to a thickness between 50 nm and 5 microns oversaid semiconductor contact layer such that the reflectivity of thereflective metal layer is greater than 80% in the wavelength range850-2000 nm and depositing a contact metal layer composed of one or morelayers of Ag, Au, and Ti on said reflective metal layer.

In some embodiments, the diffusion barrier layer is composed of one ormore of the following or alloys thereof: Cr, Pd, Pt, Si, Ti, or TiNdeposited to an aggregate thickness between 0.1 and 10.0 nm.

In another aspect, the present disclosure provides a solar cellcomprising: an epitaxial sequence of layers of III-V compoundsemiconductor material forming at least a top light-facing solar subcelland a bottom solar subcells, the bottom solar subcell having a topsurface and a bottom surface a reflective metal layer composed of one ormore of the following metals or alloys thereof: Al, Be, and Ni such thatthe reflectivity of the reflective metal layer is greater than 80%reflectivity in the wavelength range 850-2000 nm and having a thicknessbetween 50 nm and 5 microns deposited on the bottom surface of layer ofthe bottom solar subcell a contact metal layer composed of one or morelayers of Ag, Au, and Ti, having a top surface mounted to the reflectivemetal layer, and a bottom surface and a supporting substrate bonded tothe bottom surface of the contact metal layer.

In another aspect, the present disclosure provides a method ofmanufacturing a solar cell by: providing a first substrate; depositingon the first substrate a first sequence of layers of semiconductormaterial forming a first solar subcell, a second solar subcell, and athird solar subcell; depositing on said third solar subcell a gradinginterlayer; depositing on said grading interlayer a second sequence oflayers of semiconductor material forming a fourth solar subcell, thefourth solar subcell being lattice mismatched to the third solarsubcell; depositing a reflective metal layer on the fourth solarsubcell; and removing the first substrate.

In one or more embodiments, the first graded interlayer may becompositionally graded to lattice match the third solar subcell on oneside and the fourth solar subcell on the other side.

In one or more embodiments, the first graded interlayer may be composedof any of the As, P, N, Sb based III-V compound semiconductors subjectto the constraints of having the in-plane lattice parameter greater thanor equal to that of the third solar subcell and less than or equal tothat of the fourth solar subcell, and may have a band gap energy greaterthan that of the third solar subcell and of the fourth solar subcell.

In one or more embodiments, the first graded interlayer may be composedof (In_(x)Ga_(1-x))_(y)Al_(1-y)As with 0<x<1, 0<y<1, and x and yselected such that the band gap remains constant throughout itsthickness.

In one or more embodiments, the band gap of the first graded interlayermay remain at a constant value in the range of 1.42 to 1.60 eVthroughout its thickness.

In one or more embodiments, the upper first subcell may be composed ofan AlInGaP or InGaP emitter layer and an AlInGaP base layer, the secondsubcell may be composed of InGaP emitter layer and a AlGaAs base layer,the third subcell may be composed of an InGaP or GaAs emitter layer andan GaAs base layer, and the bottom fourth subcell may be composed of anInGaAs base layer and an InGaAs emitter layer lattice matched to thebase.

In one or more embodiments, the fourth solar subcell may have a band gapin the range of approximately 1.05 to 1.15 eV, the third solar subcellmay have a band gap in the range of approximately 1.40 to 1.42 eV, thesecond solar subcell may have a band gap in the range of approximately1.65 to 1.78 eV and the first solar subcell may have a band gap in therange of 1.92 to 2.2 eV.

In one or more embodiments, the fourth solar subcell may have a band gapof approximately 1.10 eV, the third solar subcell may have a band gap inthe range of 1.40-1.42 eV, the second solar subcell may have a band gapof approximately 1.73 eV and the first solar subcell may have a band gapof approximately 2.10 eV.

In one or more embodiments, the first solar subcell may be composed ofAlGaInP, the second solar subcell may be composed of an InGaP emitterlayer and a AlGaAs base layer, the third solar subcell may be composedof GaAs or InGaAs (with the value of x in In_(x) between 0 and 1%), andthe fourth solar subcell may be composed of InGaAs.

In one or more embodiments, each of the second subcell and the upperfirst subcell comprise aluminum in addition to other semiconductorelements.

In one or more embodiments, each of the second subcell and the upperfirst subcell comprise aluminum in such quantity so that the averageband gap of the top four subcells (i.e. the sum of the band gaps of eachsubcell, divided by four) is greater than 1.44 eV.

In one or more embodiments, the selection of the composition of thesubcells and their band gaps maximizes the efficiency of the solar cellat a predetermined high temperature value (in the range of 40 to 70degrees Centigrade) in deployment in space at AM0 at a predeterminedtime after the beginning of life (BOL), such predetermined time beingreferred to as the end-of-life (EOL) time.

In one or more embodiments, the selection of the composition of thesubcells and their band gaps maximizes the efficiency of the solar cellat a predetermined high temperature value (in the range of 40 to 70degrees Centigrade) not at initial deployment, but after continuousdeployment of the solar cell in space at AM0 at a predetermined timeafter the initial deployment, such time being at least one year, withthe average band gap of the top four subcells being greater than 1.44eV.

In one or more embodiments, the predetermined time after the initialdeployment is at least two years.

In some embodiments, the predetermined time is at least two years.

In some embodiments, the predetermined time is at least five years.

In some embodiments, the predetermined time is at least ten years.

In some embodiments, the predetermined time is at least twelve years.

In some embodiments, the predetermined time is at least fifteen years.

In one or more embodiments, the selection of the composition of thesubcells and their band gaps maximizes the efficiency of the solar cellat a predetermined high temperature value (in the range of 40 to 70degrees Centigrade) not at initial deployment, but after continuousdeployment of the solar cell in space at AM0 at a predetermined timeafter the initial deployment, such time being at least x years, where xis in the range of 1 to 20, with the average band gap of the top foursubcells being greater than 1.44 eV, or in some embodiments greater than1.35 eV.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teaching herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWING

The apparatus and methods described herein will be better and more fullyappreciated by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1A is a cross-sectional view of a four Junction solar cell after aninitial stage of fabrication including the deposition of certainsemiconductor layers on the growth substrate; according to the presentdisclosure;

FIG. 1B is a cross-sectional view of the solar cell of FIG. 1A with theorientation of the metal contact layer being at the bottom of theFigure;

FIG. 2A is a cross-sectional view of a five junction solar cell after aninitial stage of fabrication including the deposition of certainsemiconductor layers on the growth substrate; according to the presentdisclosure;

FIG. 2B is a cross-sectional view of the solar cell of FIG. 2A with theorientation of the metal contact layer being at the bottom of thefigure;

FIG. 3 is a transmission electron microscope image of a test solar cellwith a deposited reflective layer;

FIG. 4 is a graph that shows the reflectivity of light in a test solarcell as a function of wavelength before and after processing whichentails the application of heat to the solar cell;

FIG. 5 is a graph that illustrates the reflectivity of light in a testsolar cell with a thin mirror as a function of the wavelength of theincident light before and after processing involving the application ofheat; and

FIG. 6 is a graph that illustrates the spectral reflectivity ofperfectly smooth metal surface for a sample of metals employed in thepresent disclosure.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(sc)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Lattice matched” refers to two adjacently disposed materials havingsubstantially the same lattice constants.

“Lattice mismatched” refers to two adjacently disposed materials havingdifferent lattice constants from one another.

“Low intensity low temperature (LILT)” environment refers to a lightintensity being less than 0.1 suns, and temperatures being in the rangeless than minus 100 degrees Centigrade.

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Short circuit current (J_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero measured, for example, in milliamps.

“Short circuit current density”—see “current density”.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells which are substantially identical (i.e.plus or minus 1%).

“ZTJ” refers to a commercially available SolAero Technologies Corp.triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Details of the present disclosure will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells andinverted metamorphic multijunction solar cells are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with the solarcells of the present disclosure. However, more particularly, the presentdisclosure is directed, in various aspects, to a particular arrangementof semiconductor layers to provide a novel multijunction solar cells andsolar cell assemblies.

A variety of different features of multijunction solar cells (includinginverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. In some implementations, some or allof such features may be included in the structures and processesassociated with the lattice matched or “upright” solar cell assembliesof the present disclosure.

The present disclosure may be adapted to inverted metamorphicmultijunction solar cells that include, for example, three, four, five,or six subcells.

The present disclosure describes a process for the fabrication ofmultijunction solar cells that, in some instances, improve light capturein the associated subcell and thereby the overall efficiency of thesolar cell. More specifically, the present disclosure describes arelatively simple and reproducible technique that is suitable for use ina high volume production environment in which various semiconductorlayers are deposited in an MOCVD reactor, and subsequent processingsteps are defined and selected to minimize any physical damage to thequality of the deposited layers, thereby ensuring a relatively highyield of operable solar cells meeting specifications at the conclusionof the fabrication processes.

Prior to describing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and in particular inverted metamorphic solarcells, and the context of the composition or deposition of variousspecific layers in embodiments of the product as specified and definedby the Applicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes, such an“optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and the ultimatesolar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a “result effective variable” that one skilled in theart can simply specify and incrementally adjust to a particular leveland thereby increase the efficiency of a solar cell. The efficiency of asolar cell is not a simple linear algebraic equation as a function ofthe amount of gallium or aluminum or other element in a particularlayer. The growth of each of the epitaxial layers of a solar cell in anMOCVD reactor is a non-equilibrium thermodynamic process withdynamically changing spatial and temporal boundary conditions that isnot readily or predictably modeled. The formulation and solution of therelevant simultaneous partial differential equations covering suchprocesses are not within the ambit of those of ordinary skill in the artin the field of solar cell design.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, let alonewhether it can be fabricated in a reproducible high volume manner withinthe manufacturing tolerances and variability inherent in the productionprocess, and necessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and unexpected results, and constitute an “inventivestep”.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a vapordeposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE),Metal Organic Chemical Vapor Deposition (MOCVD), or other vapordeposition methods for the growth may enable the layers in themonolithic semiconductor structure forming the cell to be grown with therequired thickness, elemental composition, dopant concentration andgrading and conductivity type.

The growth processes for the solar cells described here can use, forexample, a MOCVD process in a standard, commercially available reactorsuitable for high volume production. The processes can be particularlysuitable for producing commercially viable multijunction solar cells orinverted metamorphic multijunction solar cells using commerciallyavailable equipment and established high-volume fabrication processes,as contrasted with merely academic expositions of laboratory orexperimental results.

Layers of a certain target composition in a semiconductor structuregrown in an MOCVD process are inherently physically different than thelayers of an identical target composition grown by another process, e.g.Molecular Beam Epitaxy (MBE). The material quality (i.e., morphology,stoichiometry, number and location of lattice traps, impurities, andother lattice defects) of an epitaxial layer in a semiconductorstructure is different depending upon the process used to grow thelayer, as well as the process parameters associated with the growth.MOCVD is inherently a chemical reaction process, while MBE is a physicaldeposition process. The chemicals used in the MOCVD process are presentin the MOCVD reactor and interact with the wafers in the reactor, andaffect the composition, doping, and other physical, optical andelectrical characteristics of the material. For example, the precursorgases used in an MOCVD reactor (e.g. hydrogen) are incorporated into theresulting processed wafer material, and have certain identifiableelectro-optical consequences which are more advantageous in certainspecific applications of the semiconductor structure, such as inphotoelectric conversion in structures designed as solar cells. Suchhigh order effects of processing technology do result in relativelyminute but actually observable differences in the material quality grownor deposited according to one process technique compared to another.Thus, devices fabricated at least in part using an MOCVD reactor orusing a MOCVD process have inherent different physical materialcharacteristics, which may have an advantageous effect over theidentical target material deposited using alternative processes.

In some cases, the solar cell according to the present application andpresent disclosure can provide increased photoconversion efficiency in amultijunction solar cell for outer space or other applications over theoperational life of the photovoltaic power system.

A 33% efficient quadruple-junctionInGaP₂/GaAs/In_(0.30)Ga_(0.70)As/In_(0.60)Ga_(0.40)As (with band gaps1.91 eV/1.42 eV/1.03 eV/0.70 eV, respectively) inverted metamorphicmultijunction cell may be 10% (relative) more efficient at beginning oflife (BOL) than standard ZTJ triple-junction devices and have 40% lowermass when permanently bonded to a 150 um thick low-mass rigid substrate.Further, inverted metamorphic technology may extend the choice ofmaterials that can be integrated together by making possiblesimultaneous realization of high quality materials that are bothlattice-matched to the substrate (InGaP and GaAs, grown first) andlattice-mismatched (In_(0.30)Ga_(0.70)As and In_(0.60)Ga_(0.40)As). Theadvantage of a metamorphic approach may be that a wide range of infraredbandgaps may be accessed via InGaAs subcells grown atop opticallytransparent step graded buffer layers. Further, metamorphic materialsmay offer near-perfect quantum efficiencies, favorably low E_(g)-V_(oc)offsets, and high efficiencies. As may often be the case though,efficiency gains may rarely materialize without additional costs. Forexample, a quadruple-junction (or “4J”) inverted metamorphicmultijunction cell may be more costly than a ZTJ due to thicker epitaxyand more complicated processing. Further, an inverted epitaxial foil maybe removed from the growth substrate and temporarily or permanentlybonded to a rigid substrate right-side-up to complete frontsideprocessing. Still further, the result may be an all-top-contact cellthat may be largely indistinguishable from a traditional ZTJ solar cell.Yet despite the quadruple-junction inverted metamorphic multijunctioncell being a higher efficiency, lower mass drop-in replacement for ZTJ,the higher specific cost [measured in $/Watt] may discourage customersfrom adopting new or incrementally modified cell technologies.

The inverted metamorphic quadruple-junction AlInGaP/AlGaAs/GaAs/InGaAs(with band gaps 2.1 eV/1.73 eV/1.42 eV/1.10 eV respectively) solar cell,or the five-junction AlInGaP/AlGaAs/GaAs/InGaAs/InGaAs (with band gaps2.1 eV/1.73 eV/1.42 eV/1.10 eV/0.90 eV) according to the presentdisclosure, is not a design that agrees with the conventional wisdom inthat an optimized multijunction cell should have balanced photocurrentgeneration among all subcells and use the entire solar spectrumincluding the infrared spectrum from 1200 nm-2000 nm. In thisdisclosure, a high bandgap current-matched triple-junction stack may begrown first followed by a lattice-mismatched 1.10 eV InGaAs subcell,which in one embodiment, forms the “bottom” subcell. The inverted InGaAssubcell is subsequently removed from the growth substrate and bonded toa rigid carrier so that the four junction or five junction solar cellcan then be processed as a normal solar cell.

Despite the beginning of life (BOL) efficiency being lower than thetraditional inverted metamorphic quadruple-junction solar cell, whenhigh temperature end of life (hereinafter referred to as “HT-EOL”) $/Wis used as the design metric, the proposed structure may provide a 10%increase in HT-EOL power and a significant decrease in HT-EOL $/W.

The proposed technology differs from existing art (e.g., U.S. Pat. No.8,969,712 B2) in that a four junction device is constructed as describedin the parent application using three lattice-matched subcells and onelattice-mismatched subcell, and three lattice matched subcells and twolattice-mismatched subcells. Previous inverted metamorphicquadruple-junction solar cells devices were constructed using twolattice-matched subcells and two lattice mismatched subcells. As aresult, the cost of the epitaxy of the proposed architecture may becheaper as the cell, e.g., may use a thinner top cell reducing In and Pusage, may reduce the number of graded buffer layers to one from two,and may eliminate the need for a high In content bottom cell, which maybe expensive due to the quantity of In required.

The basic concept of fabricating an inverted metamorphic multijunction(IMM) solar cell is to grow the subcells of the solar cell on asubstrate in a “reverse” sequence. That is, the high band gap subcells(i.e. subcells with band gaps in the range of 1.9 to 2.3 eV), whichwould normally be the “top” subcells facing the solar radiation, aregrown epitaxially on a semiconductor growth substrate, such as forexample GaAs or Ge, and such subcells are therefore lattice-matched tosuch substrate. One or more lower band gap middle subcells (i.e. withband gaps in the range of 1.3 to 1.9 eV) can then be grown on the highband gap subcells.

At least one lower subcell is formed over the middle subcell such thatthe at least one lower subcell is substantially lattice-mismatched withrespect to the growth substrate and such that the at least one lowersubcell has a third lower band gap (e.g., a band gap in the range of 0.8to 1.2 eV). A surrogate substrate or support structure is then attachedor provided over the “bottom” or substantially lattice-mismatched lowersubcell, and the growth semiconductor substrate is subsequently removed.(The growth substrate may then subsequently be re-used for the growth ofa second and subsequent solar cells).

A variety of different features of inverted metamorphic multijunctionsolar cells are disclosed in the related applications noted above. Someor all of such features may be included in the structures and processesassociated with the solar cells of the present disclosure. However, moreparticularly, the present disclosure is directed to the fabrication of afour junction inverted metamorphic solar cell using two differentmetamorphic layers, all grown on a single growth substrate. In thepresent disclosure, the resulting construction includes five subcells,with band gaps in the range of 1.92 to 2.2 eV (e.g., 2.10 eV), 1.65 to1.78 eV (e.g., 1.73 eV), 1.42 to 1.50 eV (e.g., 1.42 eV), 1.05 to 1.15eV (e.g., 1.10 eV), and 0.8 to 0.9 eV respectively.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a vapordeposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE),Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy(MBE), or other vapor deposition methods for the reverse growth mayenable the layers in the monolithic semiconductor structure forming thecell to be grown with the required thickness, elemental composition,dopant concentration and grading and conductivity type.

FIG. 1A depicts a four junction multijunction solar cell according to afirst embodiment of the present disclosure after the sequentialformation of the four subcells A, B, C, and D on a GaAs growthsubstrate. More particularly, there is shown a growth substrate 101,which is preferably gallium arsenide (GaAs), but may also be germanium(Ge) or other suitable material. For GaAs, the substrate is preferably a15° off-cut substrate, that is to say, its surface is orientated 15° offthe (100) plane towards the (111)A plane, as more fully described inU.S. Patent Application Pub. No. 2009/0229662 A1 (Stan et al.).

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the substrate 101. On the substrate, or over thenucleation layer (in the case of a Ge substrate), a buffer layer 102 andan etch stop layer 103 are further deposited. In the case of GaAssubstrate, the buffer layer 102 is preferably GaAs. In the case of Gesubstrate, the buffer layer 102 is preferably InGaAs. A contact layer104 of GaAs is then deposited on layer 103, and a window layer 105 ofAlInP is deposited on the contact layer. The subcell A, consisting of ann+ emitter layer 106 and a p-type base layer 107, is then epitaxiallydeposited on the window layer 105. The subcell A is generally latticedmatched to the growth substrate 101.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and bandgap requirements,wherein the group III includes boron (B), aluminum (Al), gallium (Ga),indium (In), and thallium (T). The group IV includes carbon (C), silicon(Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N),phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi).

In one embodiment, the emitter layer 106 is composed of InGa(Al)P₂ andthe base layer 107 is composed of InGa(Al)P₂. The aluminum or Al term inparenthesis in the preceding formula means that Al is an optionalconstituent, and in this instance may be used in an amount ranging from0% to 40%.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present disclosure to be described hereinafter.

On top of the base layer 107 a back surface field (“BSF”) layer 108preferably p+ AlGaInP is deposited and used to reduce recombinationloss.

The BSF layer 108 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, a BSF layer 108 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 108 is deposited a sequence of heavily dopedp-type and n-type layers 109 a and 109 b that forms a tunnel diode,i.e., an ohmic circuit element that connects subcell A to subcell B.Layer 109 a is preferably composed of p++ AlGaAs, and layer 109 b ispreferably composed of n++ InGaP.

A window layer 110 is deposited on top of the tunnel diode layers 109a/109 b, and is preferably n+ InGaP. The advantage of utilizing InGaP asthe material constituent of the window layer 110 is that it has an indexof refraction that closely matches the adjacent emitter layer 111, asmore fully described in U.S. Patent Application Pub. No. 2009/0272430 A1(Cornfeld et al.). The window layer 110 used in the subcell B alsooperates to reduce the interface recombination loss. It should beapparent to one skilled in the art, that additional layer(s) may beadded or deleted in the cell structure without departing from the scopeof the present disclosure.

On top of the window layer 110 the layers of subcell B are deposited:the n-type emitter layer 111 and the p-type base layer 112. These layersare preferably composed of InGaP and AlInGaAs respectively (for a Gesubstrate or growth template), or InGaP and AlGaAs respectively (for aGaAs substrate), although any other suitable materials consistent withlattice constant and bandgap requirements may be used as well. Thus,subcell B may be composed of a GaAs, InGaP, AlGaInAs, AlGaAsSb, GaInAsP,or AlGaInAsP, emitter region and a GaAs, InGaP, AlGaInAs, AlGaAsSb,GaInAsP, or AlGaInAsP base region.

In previously disclosed implementations of an inverted metamorphic solarcell, the second subcell or subcell B or was a homostructure. In thepresent disclosure, similarly to the structure disclosed in U.S. PatentApplication Pub. No. 2009/0078310 A1 (Stan et al.), the second subcellor subcell B becomes a heterostructure with an InGaP emitter and itswindow is converted from InAlP to AlInGaP. This modification reduces therefractive index discontinuity at the window/emitter interface of thesecond subcell, as more fully described in U.S. Patent Application Pub.No. 2009/0272430 A1 (Cornfeld et al.). Moreover, the window layer 110 ispreferably is doped three times that of the emitter 111 to move theFermi level up closer to the conduction band and therefore create bandbending at the window/emitter interface which results in constrainingthe minority carriers to the emitter layer.

On top of the cell B is deposited a BSF layer 113 which performs thesame function as the BSF layer 109. The p++/n++ tunnel diode layers 114a and 114 b respectively are deposited over the BSF layer 113, similarto the layers 109 a and 109 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 114 a is preferably composedof p++ AlGaAs, and layer 114 b is preferably composed of n++ InGaP.

A window layer 118 preferably composed of n+ type GaInP is thendeposited over the tunnel diode layer 114. This window layer operates toreduce the recombination loss in subcell “C”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 118, the layers of cell C are deposited: then+ emitter layer 119, and the p-type base layer 120. These layers arepreferably composed of n+ type GaAs and n+ type GaAs respectively, or n+type InGaP and p type GaAs for a heterojunction subcell, althoughanother suitable materials consistent with lattice constant and bandgaprequirements may be used as well.

In some embodiments, subcell C may be (In)GaAs with a band gap between1.40 eV and 1.42 eV. Grown in this manner, the cell has the same latticeconstant as GaAs but has a low percentage of Indium 0%<In<1% to slightlylower the band gap of the subcell without causing it to relax and createdislocations. In this case, the subcell remains lattice matched, albeitstrained, and has a lower band gap than GaAs. This helps improve thesubcell short circuit current slightly and improve the efficiency of theoverall solar cell.

In some embodiments, the third subcell or subcell C may have quantumwells or quantum dots that effectively lower the band gap of the subcellto approximately 1.3 eV. All other band gap ranges of the other subcellsdescribed above remain the same. In such embodiment, the third subcellis still lattice matched to the GaAs substrate. Quantum wells aretypically “strain balanced” by incorporating lower band gap or largerlattice constant InGaAs (e.g. a band gap of ˜1.3 eV) and higher band gapor smaller lattice constant GaAsP. The larger/smaller atomiclattices/layers of epitaxy balance the strain and keep the materiallattice matched.

A BSF layer 121, preferably composed of InGaAlAs, is then deposited ontop of the cell C, the BSF layer performing the same function as the BSFlayers 108 and 113.

The p++/n++ tunnel diode layers 122 a and 122 b respectively aredeposited over the BSF layer 121, similar to the layers 114 a and 114 b,forming an ohmic circuit element to connect subcell C to subcell D. Thelayer 122 a is preferably composed of p++ GaAs, and layer 122 b ispreferably composed of n++ GaAs.

An alpha layer 123, preferably composed of n-type GaInP, is depositedover the tunnel diode 122 a/122 b, to a thickness of about 1.0 micron.Such an alpha layer is intended to prevent threading dislocations frompropagating, either opposite to the direction of growth into the top andmiddle subcells A, B and C, or in the direction of growth into thesubcell D, and is more particularly described in U.S. Patent ApplicationPub. No. 2009/0078309 A1 (Cornfeld et al.).

A metamorphic layer (or graded interlayer) 124 is deposited over thealpha layer 123 using a surfactant. Layer 124 is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant, so as to achieve a gradualtransition in lattice constant in the semiconductor structure fromsubcell C to subcell D while minimizing threading dislocations fromoccurring. The band gap of layer 124 is constant throughout itsthickness, preferably approximately equal to 1.5 to 1.6 eV, or otherwiseconsistent with a value slightly greater than the band gap of the middlesubcell C. One embodiment of the graded interlayer may also be expressedas being composed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with x and yselected such that the band gap of the interlayer remains constant atapproximately 1.5 to 1.6 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer 124, asuitable chemical element is introduced into the reactor during thegrowth of layer 124 to improve the surface characteristics of the layer.In the preferred embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 124, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 124.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded InGaP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different bandgap. In one of the embodiments of the presentdisclosure, the layer 124 is composed of a plurality of layers ofInGaAlAs, with monotonically changing lattice constant, each layerhaving the same band gap, approximately in the range of 1.5 to 1.6 eV.

The advantage of utilizing a constant bandgap material such as InGaAlAsis that arsenide-based semiconductor material is much easier to processin standard commercial MOCVD reactors, while the small amount ofaluminum assures radiation transparency of the metamorphic layers.

Although the preferred embodiment of the present disclosure utilizes aplurality of layers of InGaAlAs for the metamorphic layer 124 forreasons of manufacturability and radiation transparency, otherembodiments of the present disclosure may utilize different materialsystems to achieve a change in lattice constant from subcell C tosubcell D. Thus, the system of Wanlass using compositionally gradedInGaP is a second embodiment of the present disclosure. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, P, N, Sb based III-Vcompound semiconductors subject to the constraints of having thein-plane lattice parameter greater than or equal to that of the secondsolar cell and less than or equal to that of the third solar cell, andhaving a bandgap energy greater than that of the second solar cell.

An alpha layer 125, preferably composed of n+ type AlGaInAsP, isdeposited over metamorphic buffer layer 124, to a thickness of about 1.0micron. Such an alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the top and middle subcells A, B and C, or in the directionof growth into the subcell D, and is more particularly described in U.S.Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

A window layer 126 preferably composed of n+ type InGaAlAs is thendeposited over alpha layer 125. This window layer operates to reduce therecombination loss in the fourth subcell “D”. It should be apparent toone skilled in the art that additional layers may be added or deleted inthe cell structure without departing from the scope of the presentdisclosure.

On top of the window layer 126, the layers of cell D are deposited: then+ emitter layer 127, and the p-type base layer 128. These layers arepreferably composed of n+ type InGaAs and p type InGaAs respectively, orn+ type InGaP and p type InGaAs for a heterojunction subcell, althoughanother suitable materials consistent with lattice constant and bandgaprequirements may be used as well.

A BSF layer 129, preferably composed of p+ type InGaAlAs, is thendeposited on top of the cell D, the BSF layer performing the samefunction as the BSF layers 108, 113 and 121.

Finally, a high band gap contact layer 130, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 129.

The composition of this contact layer 130 located at the bottom(non-illuminated) side of the lowest band gap photovoltaic cell (i.e.,subcell “D” in the depicted embodiment) in a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (i) the backside ohmic metal contact layerbelow it (on the non-illuminated side) will also act as a mirror layer,and (ii) the contact layer doesn't have to be selectively etched off, toprevent absorption.

In some embodiments, a diffusion barrier layer 131 composed of one ormore layers of either Cr, Pd, Pt, Si, Ti, or TiN to a thickness from 0.1to 10 nm is deposited on the semiconductor contact layer 130. In someembodiments the thickness of such layer may be (i) from 0.1 to 2.5 nm,or (ii) from 0.1 to 5 nm.

A reflective metal layer 132 is then deposited on the diffusion barrierlayer 131, or in some embodiments directly on the semiconductor contactlayer 130. The reflective metal layer 132 is composed of any one or moreof the following metals or alloys thereof: Al, Be, and Ni is depositedto a thickness between 50 nm and 5 microns over said semiconductorcontact layer, (or in some embodiments one or more of Ag, Al, An, Be,Cu, Mo and Ni or alloys thereof), such that the reflectivity of thereflective metal layer is greater than 80% in the wavelength range850-2000 nm.

A contact metal layer 133 composed of one or more layers of Ag, Au, andTi, is then deposited on the reflective metal layer 132. A surrogatesubstrate is then mounted on top of the contact metal layer 133, and thegrowth substrate is removed, as will be illustrated in FIG. 1B.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

FIG. 1B is a cross-sectional view of the solar cell of FIG. 1A with theorientation of the metal contact layer being at the bottom of the Figureso that the subcell A is the light-facing or top subcell of the solarcell. The figure also illustrates the surrogate or supporting substrate135 mounted or bonded to the contact metal layer 133 by an adhesive orother bonding material 134.

The adhesive layer (e.g., Wafer Bond, manufactured by or supportingBrewer Science, Inc. of Rolla, Mo.) can be deposited over the metallayer 131, and a surrogate or supporting substrate 135 can thereby beattached to the contact metal layer 133. In some embodiments, thesupporting substrate may be sapphire or glass. In other embodiments, thesurrogate or supporting substrate may be GaAs, Ge or Si, or othersuitable material. The surrogate substrate can be about 40 mils inthickness, and can be perforated with holes about 1 mm in diameter,spaced 4 mm apart, to aid in subsequent removal of the adhesive and thesubstrate. As an alternative to using an adhesive layer 134, a suitablesubstrate (e.g., GaAs) may be eutectically or permanently bonded to themetal layer 131.

The growth substrate 101 can be removed by a sequence of lapping and/oretching steps in which the substrate 101, and the buffer layer 102 areremoved. The choice of a particular etchant is growth substratedependent.

FIG. 1B depicts the growth substrate 101 having been removed. Inaddition, the buffer layer 102 and etch stop layer 103 have beenremoved, for example, by using a HCl/H2O solution.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present disclosure. For example, one or moredistributed Bragg reflector (DBR) layers can be added for variousembodiments of the present invention, and FIG. 1B illustrates thepositioning of DBR layers 150 between the BSF layer 113 and the tunneldiode layers 114 a/114 b.

FIG. 2A is a cross-sectional view of a five junction solar cell after aninitial stage of fabrication including the deposition of certainsemiconductor layers on the growth substrate; according to the presentdisclosure. In FIG. 2A layers 101, 102 . . . through 129 aresubstantially the same as that depicted in the corresponding layer ofFIG. 1A, so the description of such layers will not be repeated here forbrevity.

Turning to the new layers in FIG. 2A, the p++/n++ tunnel diode layers230 a and 230 b respectively are deposited over the BSF layer 129,forming an ohmic circuit element to connect the fourth subcell D thefifth to subcell E. The layer 230 a is preferably composed of p++AlGaInAs, and layer 230 b preferably composed of n++ GaInP.

In some embodiments an alpha layer 231, preferably composed of n-typeGaInP, is deposited over the tunnel diode 230 a/230 b, to a thickness ofabout 0.5 micron. Such alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the middle subcells C and D, or in the direction of growthinto the subcell E, and is more particularly described in copending U.S.patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

A window layer 234 preferably composed of n+ type GaInP is thendeposited over the barrier layer 233. This window layer operates toreduce the recombination loss in the fifth subcell “E”. It should beapparent to one skilled in the art that additional layers may be addedor deleted in the cell structure without departing from the scope of thepresent invention.

On top of the window layer 234, the layers of cell E are deposited: then+ emitter layer 235, and the p-type base layer 236. These layers arepreferably composed of n+ type GaInAs and p type GaInAs respectively,although other suitable materials consistent with lattice constant andband gap requirements may be used as well.

A BSF layer 237, preferably composed of p+ type AlGaInAs, is thendeposited on top of the cell E, the BSF layer performing the samefunction as the BSF layers 108, 113, 121, and 129.

Finally, a high band gap contact layer 238, preferably composed of p++type AlGaInAs, is deposited on the BSF layer 237.

In some embodiments, a diffusion barrier layer 239 composed of one ormore layers of either Cr, Pd, Pt, Si, Ti, or TiN to a thickness in therange of 2.0-4.0 nm is deposited on the semiconductor contact layer 238.

A reflective metal layer 240 is then deposited on the diffusion barrierlayer 239, or in some embodiments directly on the semiconductor contactlayer 238. The reflective metal layer 240 is composed of any one or moreof the following metals or alloys thereof: Al, Au, Cu, Mo, Be, and Ni,and is deposited to a thickness between 50 nm and 5 microns over saidsemiconductor contact layer 238, such that the reflectivity of thereflective metal layer is greater than 80% in the wavelength range of850-2000 nm.

A contact metal layer 241 composed of one or more layers of Ag, Au, andTi, is then deposited on the reflective metal layer 132. A surrogate orsupporting substrate 243 is then mounted on top of the contact metallayer 241 and the growth substrate is removed, as will be illustrated inFIG. 2B. The attachment of the surrogate or supporting substrate 243 issimilar to that of the supporting substrate 135 of FIG. 1A, so suchdescription will not be repeated here for brevity.

FIG. 2B is a cross-sectional view of the solar cell of FIG. 2A, with theorientation with the metal contact layer 241 being at the bottom of theFigure and with the growth substrate 101 having been removed. Inaddition, the etch stop layer 103 has been removed, for example, byusing a HCl/H₂O solution.

FIG. 3 is a transmission electron microscope image of a test solar cellwith a deposited reflective layer (labelled “Mirror”) composed of gold.Evidence of the diffusion of constituent atoms of gold form thereflective layer into the adjacent layers of the solar cell isindicated, demonstrating the disadvantage of utilizing of certainmaterials suggested in Cornfeld U.S. Patent Application Publication2011004898. Such diffusion would impair the material quality andefficiency of the subcell, and thereby the entire solar cell.

FIG. 4 is a graph that shows the change in reflectivity of a test mirrorin a test solar cell before processing (i.e., pre-annealing) and afterprocessing (post-annealing) of the solar cell as a function of diffusionbarrier layer thickness. It is evident that without use of a diffusionbarrier (i.e. a diffusion barrier thickness of 0.0 nm) the reflectivityof the test mirror drops from over 80% to about 50%. The graph alsoindicates that the thickness of the diffusion barrier layer also has astrong effect on reflectivity for a thickness of such layer greater than2.5 nm.

FIG. 5 is a graph that shows the reflectivity of light in a test solarcell with a thin mirror as a function of wavelength before and afterprocessing which entails the application of heat to the solar celldenoted in the Figure as “pre bond” or “post bond”. A substantialdecrease in reflectivity from over 80% to around 40% for incident lightwith wavelengths over 1400 nm is noted.

FIG. 6 is a graph that illustrates the reflectivity of certain smoothmetal surfaces as a function of the wavelength of the incident light.

As shown in reflectivity curves in FIG. 6, conventional invertedmetamorphic multijunction (IMM) solar cells have a low reflectivity inthe IR region (850-2000 nm) by virtue of the reflectivity of typicalmetals used in the back metal contacts used in these solar cells. Thismeans that about 60% of the transmitted energy in absorbed as heat atthe back metal contact. The present disclosure proposes utilizing asuitable mirror structure, composition and thickness for increasing theamount of IR light reflected back into the final junction so as toreduce the heat absorbed, thereby reducing the operating temperature ofthe solar cell which is important for optimizing the efficiency of suchsolar cells operating in high temperature space environment.

Additionally, a high reflectivity metal mirror as proposed in thepresent disclosure would result in more light energy (i.e. photons)reflected into the adjacent bottom subcell or junction which could beabsorbed by this junction and converted into electricity. Moreabsorption increases the current in this subcell's junction, which canbe translated into a 9.34% increase in Quantum Efficiency (QE) of thatsubcell according to the use of a suitable metal mirror as proposed inthe present disclosure.

Not all metals depicted in FIG. 6 (such as gold) are suitable backreflectors due to interdiffusion of the reflective metal into the solarcell junction during processing as shown in FIG. 3. Temperaturesrequired to process solar cells can lead to both a reduction inreflectivity such as demonstrated in the experimental evidence on testsolar cells depicted in FIG. 4 a result of subsequent high temperatureprocessed used in fabrication of IMM solar cells referred to as “bond”and thereby a reduction in solar cell performance for cells withreflective metals.

Two solutions are proposed here. First, comparable materials selectionand thickness that results in no deleterious diffusion. Second, theutilization of a diffusion barrier material (layer 139 in FIG. 1A).

The infrared light energy that is not absorbed by the solar cell istransmitted to the reflective back metal layer, which reflects the lightback into the adjoining subcell where it can be absorbed in thatsubcell, thereby increasing current in that subcell and potentiallylowering the operating temperature of the solar cell. Options forimplementing a reflective back mirror include (i) direct deposition ofmirror material to the cell, (ii) introduction of carriers withreflective material on the interface surface, (iii) utilizing areflective adhesive, and (iv) utilizing reflective panel materials.

The present disclosure covers different specific mirror materials whichmay have a surface which is (patterned or not patterned): composed ofone or more of the metals Ag, Au, Al, Be, Cu, Mo and Ni, or acombination thereof, deposited to a thickness between 50 nm-5 μm.Between the mirror material and the solar cell there may be deposited anoptional additional diffusion barrier material layer. The diffusionbarrier material layer can be composed of one or more of the followingmetals: Cr, Pd, Pt, Ti, or TiN deposited to a thickness of 0.1 nm-10 nm.In some embodiments a diffusion barrier layer thickness of 1.0 to 3.0 nmmay be implemented.

In addition to the implementation of a reflective back metal layer, thebaseline inverted metamorphic multijunction solar cells illustrated inthe present disclosure follows a design rule that one should incorporateas many high bandgap subcells as possible to achieve the goal toincrease high temperature EOL performance. For example, high bandgapsubcells may retain a greater percentage of cell voltage as temperatureincreases, thereby offering lower power loss as temperature increases.As a result, both HT-BOL and HT-EOL performance of the exemplaryinverted metamorphic multijunction solar cell may be expected to begreater than traditional cells.

For example, the cell efficiency (%) measured at room temperature (RT)28° C. and high temperature (HT) 70° C., at beginning of life (BOL) andend of life (EOL), for a standard three junction commercial solar cell(ZTJ) is as follows:

Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4%After 5E14 e/cm² radiation EOL 70° C. 22.0% After 1E15 e/cm² radiation

For the four junction inverted metamorphic multijunction solar celldepicted in FIGS. 1A and 1B and described in the related application,(the “IMMX”) the corresponding data (without implementing the gradedband gap of the present application) is as follows:

Condition Efficiency BOL 28° C. 29.5% BOL 70° C. 26.6% EOL 70° C. 24.7%After 5E14 e/cm² radiation EOL 70° C. 24.2% After 1E15 e/cm² radiationOne should note the slightly higher cell efficiency of the IMMX solarcell than the standard commercial solar cell (ZTJ) at BOL both at 28° C.and 70° C. However, the IMMX solar cell described above exhibitssubstantially improved cell efficiency (%) over the standard commercialsolar cell (ZTJ) at 1 MeV electron equivalent fluence of 5×10¹⁴ e/cm²,and dramatically improved cell efficiency (%) over the standardcommercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of1×10¹⁵ e/cm².

For the five junction IMMX solar cell depicted in FIGS. 2A and 2Bdescribed in one or more of the related applications, the correspondingdata (without implementing the graded band gap of the presentapplication) is as follows:

Condition Efficiency BOL 28° C. 32.1% BOL 70° C. 30.4% EOL 70° C. 27.1%After 5E14 e/cm² radiation EOL 70° C. 25.8% After 1E15 e/cm² radiation

In some embodiments, the solar cell according to the present disclosureis also applicable to low intensity (LI) and/or low temperature (LT)environments, such as might be experienced in space vehicle missions toMars, Jupiter, and beyond. A “low intensity” environment refers to alight intensity being less than 0.1 suns, and a “low temperature”environment refers to temperatures being in the range of less than minus100 degrees Centigrade.

For such applications, depending upon the specific intensity andtemperature ranges of interest, the band gaps of the subcells may beadjusted or “tuned” to maximize the solar cell efficiency, or otherwiseoptimize performance (e.g. at EOL or over the operational working lifeperiod).

In view of different satellite and space vehicle requirements in termsof operating environmental temperature, radiation exposure, andoperational life, a range of subcell designs using the design principlesof the present disclosure may be provided satisfying specific definedcustomer and mission requirements, and several illustrative embodimentsare set fourth hereunder, along with the computation of their efficiencyat the end-of-life for comparison purposes. As described in greaterdetail below, solar cell performance after radiation exposure isexperimentally measured using 1 MeV electron fluence per squarecentimeter (abbreviated in the text that follows as e/cm²), so that acomparison can be made between the current commercial devices andembodiments of solar cells discussed in the present disclosure.

As an example of different mission requirements, a low earth orbit (LEO)satellite will typically experience radiation of protons equivalent toan electron fluence per square centimeter in the range of 1×10¹² e/cm²to 2×10¹⁴ e/cm² (hereinafter may be written as “2E10 e/cm² or 2E14”)over a five year lifetime. A geosynchronous earth orbit (GEO) satellitewill typically experience radiation in the range of 5×10¹⁴ e/cm² to1×10¹⁵ e/cm² over a fifteen year lifetime.

The wide range of electron and proton energies present in the spaceenvironment necessitates a method of describing the effects of varioustypes of radiation in terms of a radiation environment which can beproduced under laboratory conditions. The methods for estimating solarcell degradation in space are based on the techniques described by Brownet al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of theTelstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963]and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G.Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication82-69, 1982]. In summary, the omnidirectional space radiation isconverted to a damage equivalent unidirectional fluence at a normalizedenergy and in terms of a specific radiation particle. This equivalentfluence will produce the same damage as that produced by omnidirectionalspace radiation considered when the relative damage coefficient (RDC) isproperly defined to allow the conversion. The relative damagecoefficients (RDCs) of a particular solar cell structure are measured apriori under many energy and fluence levels in addition to differentcover glass thickness values. When the equivalent fluence is determinedfor a given space environment, the parameter degradation can beevaluated in the laboratory by irradiating the solar cell with thecalculated fluence level of unidirectional normally incident flux. Theequivalent fluence is normally expressed in terms of 1 MeV electrons or10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculatethe specific electron and proton fluence that a solar cell is exposed toduring a specific satellite mission as defined by the duration,altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed bythe Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damageequivalent electron and proton fluences, respectively, for exposure tothe fluences predicted by the trapped radiation and solar proton modelsfor a specified mission environment duration. The conversion to damageequivalent fluences is based on the relative damage coefficientsdetermined for multijunction cells [Marvin, D. C., Assessment ofMultijunction Solar Cell Performance in Radiation Environments,Aerospace Report No. TOR-2000 (1210)-1, 2000]. New cell structureseventually need new RDC measurements as different materials can be moreor less damage resistant than materials used in conventional solarcells. A widely accepted total mission equivalent fluence for ageosynchronous satellite mission of 15 year duration is 1 MeV 1×10¹⁵electrons/cm².

The exemplary solar cell described herein may require the use ofaluminum in the semiconductor composition of each of the top two orthree subcells. Aluminum incorporation is widely known in the III-Vcompound semiconductor industry to degrade BOL subcell performance dueto deep level donor defects, higher doping compensation, shorterminority carrier lifetimes, and lower cell voltage and an increased BOLE_(g)-V_(oc) metric. In short, increased BOL E_(g)-V_(oc) may be themost problematic shortcoming of aluminum containing subcells; the otherlimitations can be mitigated by modifying the doping schedule orthinning base thicknesses.

Furthermore, at BOL, it is widely accepted that great subcells have aroom temperature E_(g)-V_(oc) of approximately 0.40. A wide variation inBOL E_(g)-V_(oc) may exist for subcells of interest to IMMX cells.However, Applicants have found that inspecting E_(g)-V_(oc) at HT-EOLmay reveal that aluminum containing subcells perform no worse than othermaterials used in III-V solar cells. For example, all of the subcells atEOL, regardless of aluminum concentration or degree of lattice-mismatch,have been shown to display a nearly-fixed E_(g)-V_(oc) of approximately0.6 at room temperature 28° C.

The exemplary inverted metamorphic multijunction solar cell designphilosophy may be described as opposing conventional cell efficiencyimprovement paths that employ infrared subcells that increase in expenseas the bandgap of the materials decreases. For example, proper currentmatching among all subcells that span the entire solar spectrum is oftena normal design goal. Further, known approaches—including dilutenitrides grown by MBE, upright metamorphic, and inverted metamorphicmultijunction solar cell designs—may add significant cost to the celland only marginally improve HT-EOL performance. Still further, lowerHT-EOL $/W may be achieved when inexpensive high bandgap subcells areincorporated into the cell architecture, rather than more expensiveinfrared subcells. The key to enabling the exemplary solar cell designphilosophy described herein is the observation that aluminum containingsubcells perform well at HT-EOL.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

The terminology used in this disclosure is for the purpose of describingspecific identified embodiments only and is not intended to be limitingof different examples or embodiments.

In the drawings, the position, relative distance, lengths, widths, andthicknesses of supports, substrates, layers, regions, films, etc., maybe exaggerated for presentation simplicity or clarity. Like referencenumerals designate like elements throughout the specification. It willbe understood that when an element such as an element layer, film,region, or feature is referred to as being “on” another element, it canbe disposed directly on the other element or the presence of interveningelements may also be possible. In contrast, when an element is referredto as being disposed “directly on” another element, there are nointervening elements present.

Furthermore, those skilled in the art will recognize that boundaries andspacings between the above described units/operations are merelyillustrative. The multiple units/operations may be combined into asingle unit/operation, a single unit/operation may be distributed inadditional units/operations, and units/operations may be operated atleast partially overlapping in time. Moreover, alternative embodimentsmay include multiple instances of a particular unit/operation, and theorder of operations may be altered in various other embodiments.

The terms “substantially”, “essentially”, “approximately”, “about”, orany other similar expression relating to particular parametric numericalvalue are defined as being close to that value as understood by one ofordinary skill in the art in the context of that parameter, and in onenon-limiting embodiment the term is defined to be within 10% of thatvalue, in another embodiment within 5% of that value, in anotherembodiment within 1% of that value, and in another embodiment within0.5% of that value.

The term “coupled” as used herein is defined as connected, although notnecessarily directly or physically adjoining, and not necessarilystructurally or mechanically. A device or structure that is “configured”in a certain way is arranged or configured in at least that describedway, but may also be arranged or configured in ways that are notdescribed or depicted.

The terms “front”, “back”, “side”, “top”, “bottom”, “over”, “on”,“above”, “beneath”, “below”, “under”, and the like in the descriptionand the claims, if any, are used for descriptive purposes and notnecessarily for describing permanent relative positions. It isunderstood that the terms so used are interchangeable under appropriatecircumstances such that the embodiments of the disclosure describedherein are, for example, capable of operation in other orientations thanthose illustrated or otherwise described herein. For example, if theassembly in the figures is inverted or turned over, elements of theassembly described as “below” or “beneath” other elements or featureswould then be oriented “above” the other elements or features. Thus, theexemplary term “below” can encompass both an orientation of above andbelow. The assembly may be otherwise oriented (rotated by a number ofdegrees through an axis).

The terms “front side” and “backside” refer to the final arrangement ofthe panel, integrated cell structure or of the individual solar cellswith respect to the illumination or incoming light incidence.

In the claims, the word ‘comprising’ or ‘having’ does not exclude thepresence of other elements or steps than those listed in a claim. It isunderstood that the terms “comprises”, “comprising”, “includes”, and“including” if used herein, specify the presence of stated components,elements, features, steps, or operations, components, but do notpreclude the presence or addition of one or more other components,elements, features, steps, or operations, or combinations andpermutations thereof.

The terms “a” or “an”, as used herein, are defined as one or more thanone. Also, the use of introductory phrases such as “at least one” and“one or more” in the claims should not be construed to imply that theintroduction of another claim element by the indefinite articles “a” or“an” limits any particular claim containing such introduced claimelement to disclosures containing only one such element, even when theclaim includes the introductory phrases “one or more” or “at least one”and indefinite articles such as “a” or “an”. The same holds true for theuse of definite articles.

The present disclosure can be embodied in various ways. To the extent asequence of steps are described, the above described orders of the stepsfor the methods are only intended to be illustrative, and the steps ofthe methods of the present disclosure are not limited to the abovespecifically described orders unless otherwise specifically stated. Notethat the embodiments of the present disclosure can be freely combinedwith each other without departing from the spirit and scope of thedisclosure.

Although some specific embodiments of the present disclosure have beendemonstrated in detail with examples, it should be understood by aperson skilled in the art that the above examples are only intended tobe illustrative but not to limit the scope and spirit of the presentdisclosure. The above embodiments can be modified without departing fromthe scope and spirit of the present disclosure which are to be definedby the attached claims. Accordingly, other implementations are withinthe scope of the claims.

Although described embodiments of the present disclosure utilizes avertical stack of a certain illustrated number of subcells, variousaspects and features of the present disclosure can apply to stacks withfewer or greater number of subcells, i.e. two junction cells, threejunction cells, four, five, six, seven junction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell C, with p-type and n-type InGaAs is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or emitter layer may also beinstrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP,AlGaInAs, AlGaInPAs, GaInP, GaINaS, GaInPAs, AlGaAs, AnInAs, AlInPAs,GaAsSb, GaAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN,GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fallwithin the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand, LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the forgoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptions should and are intended to be comprehendedwithin the meaning and range of equivalence of the following claims.

1. A method of manufacturing a solar cell comprising: providing a growthsubstrate; depositing on the growth substrate an epitaxial sequence oflayers of semiconductor material forming at least a first and secondsolar subcells; depositing a semiconductor contact layer on top of thesecond solar subcell; depositing a reflective metal layer composed ofany one or more of the following metals or alloys thereof: Al, Be, andNi to a thickness between 50 nm and 5 microns over said semiconductorcontact layer; such that the reflectivity of the reflective metal layeris greater than 80% in the wavelength range 850-2000 nm; depositing acontact metal layer composed of one or more layers of Ag, Au, and Ti onsaid reflective metal layer; mounting and bonding a surrogate substrateon top of the contact metal layer; and removing the first substrate. 2.A method as defined in claim 1, wherein the reflective metal layercomprises one or more of the following metals or alloys thereof: Ag, Al,Au, Be, Cu, Ni or Ti, and further comprising depositing a diffusionbarrier layer directly on said semiconductor contact layer, wherein thereflective metal layer is deposited directly on the diffusion barrierlayer.
 3. A method as defined in claim 2, wherein the diffusion barrierlayer is composed of one or more layers of Cr, Pd, Pt, Si, Ti or TiN. 4.A method as defined in claim 2, wherein the diffusion layer has athickness of between 0.1 nm and 10 nm.
 5. A method as defined in claim1, further comprising: depositing on the growth substrate a firstsequence of layers of semiconductor material forming a first solarsubcell, a second solar subcell, and a third solar subcell; depositingon said third solar subcell a first grading interlayer; and depositingon said first grading interlayer a second sequence of layers ofsemiconductor material forming a fourth solar subcell, the fourth solarsubcell being lattice mismatched to the third solar subcell.
 6. A methodas defined in claim 5, wherein the first graded interlayer iscompositionally graded to lattice matched the third solar subcell on oneside and the lower fourth solar subcell on the other side, and iscomposed of the As, P, N, Sb based III-V compound semiconductors subjectto the constraints of having the in-plane lattice parameter greater thanor equal to that of the third solar subcell and less than or equal tothat of the lower fourth solar subcell, and having a band gap energygreater than that of the third solar subcell and the fourth solarsubcell.
 7. A method as defined in claim 5, wherein, the fourth solarsubcell has a band gap in the range of approximately 1.05 to 1.15 eV,the third solar subcell has a band gap in the range of approximately1.40 to 1.50 eV, the second solar subcell has a band gap in the range ofapproximately 1.65 to 1.78 eV and the first solar subcell has a band fapin the range of 1.92 to 2.2 eV, and wherein the graded interlayer iscomposed of (In_(x)Ga_(1-x))Al_(1-y)As with 0<x<1, 0<y<1, and x and yselected such that the band gap remains constant throughout itsthickness, and the band gap of the graded interlayer remains at aconstant value in the range of 1.42 to 1.60 eV throughout its thickness.8. A method as defined in claim 1, wherein the upper first solar subcellis composed of AlGaInP, the second solar subcell is composed of an InGaPemitter layer and a AlGaAs base layer, the third solar subcell iscomposed of GaAs, and the lower fourth solar subcell is composed ofInGaAs.
 9. A method as defined in claim 1, further comprising: adistributed Bragg reflector (DBR) layer adjacent to and between thesecond and the third solar subcells and arranged so that light can enterand pass through the second solar subcell and at least a portion ofwhich can be reflected back into the second subcell by the DBR layer.10. A method as defined in claim 1, further comprising: a distributedBragg reflector (DBR) layer adjacent to and between the third solarsubcell and the graded interlayer and arranged so that light can enterand pass through the third solar subcell and at least a portion of whichcan be reflected back into the third solar subcell by the DBR layer. 11.A method as defined in claim 10, wherein the distributed Bragg reflectorlayer is composed of a plurality of alternating layers of latticematched materials with discontinuities in their respective indices ofrefraction.
 12. A method as defined in claim 11, wherein the differencein refractive indices between alternating layers is maximized in orderto minimize the number of periods required to achieve a givenreflectivity, and the thickness and refractive index of each perioddetermines the stop band and its limiting wavelength.
 13. A method asdefined in claim 12, wherein the DBR layer includes a first DBR layercomposed of a plurality of p type Al_(x)Ga_(1-x)As layers, and a secondDBR layer disposed over the first DBR layer and composed of a pluralityof p type Al_(y)Ga_(1-y)As layers, with 0<x<1, 0<y<1, and where y isgreater than x.
 14. A method as defined in claim 9, wherein thedistributed Bragg reflector layer is composed of a plurality ofalternating layers of lattice matched materials with discontinuities intheir respective indices of refraction.
 15. The multijunction solar cellas defined in claim 14, wherein the difference in refractive indicesbetween alternating layers in maximized in order to minimize the numberof periods required to achieve a given reflectivity, and the thicknessand refractive index of each period determines the stop band and itslimiting wavelength.
 16. A method as defined in claim 15, wherein theDBR layer includes a first DBR layer composed of a plurality of p typeAl_(x)Ga_(1-x)As layers, and a second DBR layer disposed over the firstDBR layer and composed of a plurality of p type Al_(y)Ga_(1-y)As layers,with 0<x<1, 0<y<1, and where y is greater than x.
 17. A method asdefined in claim 4, wherein the thickness of the diffusion barrier layerin between 1.0 and 3.0 nm.
 18. A method of manufacturing a solar cellcomprising: providing a growth substrate; depositing on the growthsubstrate an epitaxial sequence of layers of a III-V compoundsemiconductor material forming at least a first top or light-facingsolar subcell and a second bottom solar subcell, the second bottom solarsubcell has a top surface and a bottom surface; depositing a diffusionbarrier layer directly on the bottom surface of the bottom solarsubcell; depositing a reflective metal layer directly on the diffusionbarrier layer to a thickness between 50 nm and 5 microns over saidsemiconductor contact layer such that the reflectivity of the reflectivemetal layer is greater than 80% in the wavelength range 850-2000 nm; anddepositing a contact metal layer composed of one or more layers of Ag,Au, and Ti, on said reflective metal layer.
 19. A method as defined inclaim 18, wherein the diffusion barrier layer is composed of one or moreof the following or alloys thereof: Cr, Pd, Pt, Si, Ti, or TiN depositedto an aggregate thickness between 0.1 and 10.0 nm.
 20. A solar cellcomprising: an epitaxial sequence of layers of III-V compoundsemiconductor material forming at least a top light-facing solar subcelland a bottom solar subcell, the bottom solar subcell having a topsurface and a bottom surface; a reflective metal layer deposed directlyon the bottom surface of the bottom solar subcell composed of one ormore of the following metals or alloys thereof: Al, Be, and Ni such thatthe reflectivity of the reflective metal layer is greater than 80%reflectivity in the wavelength range of 850-2000 nm and having athickness between 50 nm and 5 microns; a contact metal layer composed ofone or more layers of Ag, Au, and Ti having a top surface deposed on thereflective metal layer, and a bottom surface; and a supporting substratebonded to the bottom surface of the contact metal layer.